I guess "good" is a bit of a relative term, but a great dictionary, which is entirely free, is Lewis & Short, and the best interface for it is through the entirely free program Diogenes (which also does a ton of other useful stuff, like parse Latin and Greek words, and includes the LSJ dictionary for ancient Greek stuff). Anyway, you can download it from the author:

He has made Windows, Mac and Linux versions. Another nice feature is that the entire thing is local on your computer, so there is no need for an active internet connection, and your lookups are instantaneous (unlike, say Perseus, where you often wait quite a long time for the result page).